The present invention relates to sheet-fed printing machines and, more particularly to an improved stack changing device for sheet feeders of sheet-fed printing machines.
It is a known practice, in sheet feeders of sheet-fed printing presses, to provide arrangements for automated stack change. These can consist of rack-type structures, so-called remaining stack carrying devices, which are provided with thrust and lift drives for the horizontal and vertical movement. Such so-called non-stop stack changers are suited, during the printing of paper sheets, for example in machine running, to remove the remaining of used-up sheet stacks from a pallet, provided for example with grooves and to deposit them again on a new sheet stack subsequently installed in the sheet feeder. Known devices are distinguished by a great expenditure in construction and assembly, and require special constructions of the sheet feeders. Further, here devices are used, the remaining stack carrying device of which requires a rake engaging into the grooves of the pallet. This rake has to be removed in the joining of the remaining stack with the newly installed sheet stack as a whole between the two stack parts. This involves high drive forces and places very strong stresses on the sheets lying next to the section point.
Furthermore, restraining means are to be provided which prevent a shifting of the stack parts, and, in the process, severely stress the stack edges. Furthermore, the operation of the sheet feeder itself is severely hindered or even made impossible. The sheet flow is difficult to control in the changing operation, so that wasted sheets result again and again.
There have already been developed devices which partly avoid some of the disadvantages described.
Thus, from DE 393 1710 C2 there is known a non-stop sheet feeder for sheet-fed rotary presses. It has a remaining-stack carrying device which is arranged underneath a band table leading from the sheet feeder to the sheet-fed rotary press. The remaining-stack device has a closed frame on which there are arranged non-stop bars, which can be driven as piston rods of individual cylinders by means of a pressure medium, and which are drivable into grooves of a pallet carrying a sheet stack. The non-stop bars lie, in the driven-in state, on both sides of the frame and are to be removed successively from the zone of the sheet feeder. While the rods have individual drive cylinders, the patent discloses nothing about the sequence of operation. The bridging of the gap conditioned by the non-stop bars between main stack and remaining stack is, an obstacle for a faultless continuous processing in the unification of the stacks.
From DE 4203500 A1 a sheet feeder is known. It presents, parallel to the sheet feeder and allocated to this on face side, an auxiliary-stack carrying device as an independent component. Then, over a common drive individually drivable pointed bars are provided which can be introduced into grooves of a pallet carrying a sheet stack. The drive has individual chain gears which are couplable onto the respective pointed bars. For the guidance and accessibility of the chain gears special constructive measures are required. The chain gears completely block the space in front of the sheet feeder, so that this latter is not accessible.
In the stack change it is provided, in the joining of main stack and remaining stack, to remove the pointed bars out of the stack zone, first on the outside, then in the middle and last of all in the zone between the already pulled pointed bars, so that a gentle depositing of the remaining stack on the sheet stack results. This is possible, however, with the requisite precision, only in the case of heavy materials such as sheets made of metal.
Finally, from DE 19520772 C1 there is known a non-stop sheet feeder for printing presses with fork bars movable in and out. In this feeder, on both sides of the stack zone, there are provided units of fork bars movable on both sides of the stack zone transversely to the sheet transport direction. The fork bars are connected with one another and drivable in common into grooves of a pallet carrying a sheet stack. From there they can take over an auxiliary stack in order to bridge the time period up to the supplying of a new sheet stack. For the preliminary approach of the main stack and of the remaining stack before the final joining, the fork bars are rectangular in cross section and turnable about a longitudinal axis. There the fork bars are thrust in first in the upright orientation, in order to be able to carry as great a load as possible. For the approach they are then rotated through 90 degrees, so that the main stack and the remaining stack approach the value of the thickness of the narrower side to each other. In the turning of the fork rods measures have to be taken in order to avoid a shifting of the fork bars. This has proved in practice to be virtually unfeasible.
Further there is known from DAS 1095297 a sheet feeder with several stack-lifting mechanisms. It has a fork-shaped remaining-stack carrying device which is provided with remaining-stack bars slidable into grooves of a pallet. The device makes possible the take-over of a remaining of a sheet stack from the pallet for the continuous feeding of the sheets while a new sheet stack is installed into the sheet feeder. The remaining-stack device is connected with a separate lifting mechanism parallel to the main-stack lifting mechanism inside the sheet feeder, so that the remaining stack is continuously liftable. The operating range of the remaining-stack carrying device is restricted. The remaining-stack carrying device hampers the access to the sheet feeder.
In view of the foregoing, an object of the present invention is to provide an improved stack changing device which overcomes the problems associated with prior art designs.
A more specific object of the present invention is to provide a stack changing device which allows for simple and continuous stack changing with undisturbed sheet transport and no wasted paper.
The present invention provides these and other advantages and overcomes the drawbacks of the prior art by providing a stack changing device which utilizes an improved method and device for receiving a remaining-stack and transferring the remaining stack to a newly fed-in sheet stack.
It is advantageous that carrying and spacing bars, independent from one another, are provided in the device, which are pulled, not simultaneously but intermittently, out of the stack zone for the unburdening of the sheet material. There preferably can be provided a different height for the two types of bars, which produces a smooth depositing movement of the stack remaining on the sheet stack.
In particular the continuous removal of the remaining stack bars from the inside outward and in two stages makes possible a gentle depositing of the remaining stack on the sheet stack. Preferably, a speed profile should be maintained for influencing the stack approach, in the pulling speeds between the individual bars.
These and other features and advantages of the invention will be more readily apparent upon reading the following description of preferred exemplary embodiments of the invention and upon reference to the drawings wherein: